This explanation is pretty close. ROBOTC relies on "edge triggered" interrupts for detecting the response from a SONAR sensor. Only the first two of the PIC microchip's six interrupts can be edge triggered.
The other four generate an interrupt on any transition. And the interrupt is "there's been a value change in one of the four interrupt pins". So it is up to the firmware to figure was it a high to low transition (that's what is desired) and on which pin. The code for this is not terribly complicated, but it is not something that has been implemented in ROBOTC firmware at present because of the low usage.